<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        #boxA {
            background-color: blue;
            padding: 5px;
        }
        #boxB {
            background-color: green;
            padding: 5px;
        }
    </style>
</head>
<body>
    <div id="boxA">
        <img src="images/2.1.a.jpg">
    </div>
    <input type="button" value="复制" onclick="doMove()">
    <input type="button" value="回去" onclick="moveBack()">
    <div id="boxB">
    </div>
    <script>
        function doMove() {
            var boxA = document.getElementById('boxA');
            var boxB = document.getElementById('boxB');
            var img = boxA.firstElementChild;
            if (img)
                boxB.appendChild(img);
        }
        function moveBack() {
            var boxA = document.getElementById('boxA');
            var boxB = document.getElementById('boxB');
            var img = boxB.firstElementChild;
            if (img)
                boxA.appendChild(img);
        }
        //思考，为什么不是复制
    </script>
</body>
</html>